1. Field of the Invention
The present invention relates to a method and an apparatus for erasing a magnetic disc and more particularly, a method and an apparatus for erasing a recording track formed concentrically on the magnetic disc used, for example, in an electronic still camera.
2. Description of the Prior Art
Recently, an electronic still camera has been developed, which is disclosed in Japanese Periodical "Nikkei Electronics", July 2, 1984, pp. 80-85. Such an electronic still camera is adapted such that a plurality of tracks are formed concentrically on a magnetic disc and signals for a field or a frame of an imaged still picture are recorded on each track.
Erasing of a recording track on the magnetic disc for the electronic still camera is generally performed by recording an erasing signal with a frequency higher than the recording frequency (0-12 MHz). However, if frequency of the erasing signal is set too high, the efficiency of erasing is reduced, so that a recording signal, particularly the low frequency component thereof remains un-erased. This is particularly caused by the fact that a recording magnetic field of the low frequency signal component reaches the deep layer of the magnetic disc.
In order to solve the problem, the following approaches have been proposed. More specifically, Japanese Patent Laying-Open Gazette No. 902/1986 discloses a first approach in which the recording track is erased by the erasing signal whose frequency is increased from an initial frequency of 2 to 3 MHz to a final frequency of about 14 MHz with time. According to the first approach, the recording magnetic field of the low frequency component is erased by the initial frequency of 2 to 3 MHz. Thereafter, as the frequency of the erasing signal becomes higher, the recording magnetic field with a higher frequency is sequentially erased. Finally, only the recording magnetic field with the final frequency of 14 MHz set outside the recording frequency band remains. As a result, recorded information is erased. However, it is difficult to form an oscillator whose frequency changes continuously from the initial frequency of 2 to 3 MHz to the final frequency of 14 MHz. In practice, it is suitable to employ an oscillator which generates the erasing signals each having a frequency such as 2, 5, 8, 11, and 14 MHz which increase in a stepping manner, as shown in FIG. 4 in the above described document. More specifically, the erasing signal of 5 MHz erases the recorded erasing signal of 2 MHz and the erasing signal of 8 MHz erases the erasing signal of 5 MHz. The point of such an erasing method is that the frequency of the first erasing signal (2 to 3 MHz in the above described example) is such that the low frequency component of the recording track can be sufficiently erased, the frequency of the n-th erasing signal is such that the (n-1)-th erasing signal can be sufficiently erased, and the frequency of the last erasing signal (14 MHz in the above described example) is outside of the recording band.
On the other hand, Japanese Patent Laying-Open Gazette No. 221805/1984 discloses a second approach in which residual distortion due to the stationary oscillation component is effectively removed by switching the erasing signal from stationary oscillation with a low frequency to damping oscillation with a high frequency at predetermined timing. Therefore, if the last erasing signal in the above described first approach is attenuated and erased as shown in the second approach, the residual value of the last erasing signal can be decreased.
However, in the erasing method according to the above described first approach, switching of the frequency from the initial frequency to the final frequency must be made over many stages. Thus, a frequency interval between the n-th erasing signal and the (n-1)-th erasing signal can not be increased, so that the structure of the apparatus can not be simplified. In addition, for the reasons described above, it is difficult to set the final frequency to a higher value, so that occurrence of the beat at the time of re-recording as described below can not be completely prevented.
In the above described second approach, the rotational speed of a driving motor of the magnetic disc must be regulated to increase the frequency at the time of damping oscillation, so that it becomes difficult to structure a circuit and switching noise occurs at the time of switching to a higher frequency.